1. Field of the Invention
The present invention relates to a solid-state image sensor that comprises a micro-lens for each photodiode.
2. Description of the Background Art
Some conventional solid-state image sensors provide the wide dynamic range of an image signal representative of a scene captured while maintaining the photo-sensitivity of photodiodes or photosensitive cells. For example, in the solid-state image sensor disclosed in Japanese Patent Laid-Open Publication No. 298175/1992, hereinafter referred to as Document 1, two types of horizontal lines of photodiodes are arranged alternately in the vertical direction. Specifically, one type of horizontal line contains higher-sensitivity photodiodes and the other contains lower-sensitivity photodiodes. Then, every two photodiodes, vertically adjacent to each other, of different sensitivities work together as one pixel. Because signal charges obtained respectively from the two photodiodes as one pixel are added up to a signal charge of the pixel, the sensor can accomplish a scene with a wide dynamic range.
Another Japanese Patent Laid-Open Publication No. 74926/1998, hereinafter referred to as Document 2, describes a solid-state image sensor comprising an array of photodiodes or photosensitive cells and micro-lenses, so-called on-chip lenses, correspondingly placed on the photodiodes toward a field of view to be captured for concentrating incident photons or beams from the field onto the respective photodiodes. The solid-state image sensor disclosed in Document 2 does not have only one type of photodiodes that receive incident beams impinging on the micro-lenses but also another type of photodiodes that receive incident beams impinging on, and transmitted through, the spaces between the lenses to increase the aperture efficiency of the respective pixels.
When one pixel is composed of two photodiodes of different sensitivities as described in Document 1, there is a layout restriction such that those two photodiodes forming one pixel must be adjacent to each other. Because of this restriction, the solid-state image sensor described in Document 1 has to have the higher-sensitivity photodiodes arranged in odd-numbered lines only and the lower-sensitivity photodiodes arranged in even-numbered lines only. Each pixel is thus formed by a couple of photodiodes existing on two horizontal lines, adjacent to each other, in the same horizontal position of the array of photodiodes. This means that the resolution in the vertical direction is degraded to one half of that achieved in the case that each pixel exists on one line in the same horizontal position.
When one pixel is composed of two photodiodes, it is better to provide micro-lenses on both of the higher- and lower-sensitivity photodiodes than utilizing the spaces between the micro-lenses placed on the higher-sensitivity photodiodes to conduct incident beams to the lower-sensitivity photodiodes as described in Document 2. More specifically, the provision of micro-lenses of a smaller diameter on the lower-sensitivity photodiodes in addition to micro-lenses of a larger diameter disposed on the higher-sensitivity photodiodes results in a higher efficiency in concentrating the beams. However, because part of the incident beams impinging on the smaller-diameter micro-lenses may be blocked by the larger-diameter micro-lenses, a shading problem could occur.